The present invention relates to a combined on-off time control and short circuit control system for a wire feed electrical discharge machining apparatus. Such apparatus employs a precisely driven and guided wire electrode for generating very small radii in intricate shapes. Because of the nature of the wire cutting operation, there are limitations to the relative movement between the wire electrode and workpiece that can be employed to break gap short circuit conditions. In conventional cavity sinking electrical discharge machining where the tool electrode is mounted on a vertical head, there are a variety of expedients used to prevent and to interrupt the formation of gap short circuit, for example by fast back-up for the head and electrode when gap short circuit occurs and also by vibration of the workpiece and/or electrode during machining to flush the gap and to further inhibit the gap short circuit condition.
The present invention as it relates to the gap short circuit part of the power supply used with a wire cutter includes provision for precisely controllable on-time, off-time and current magnitude. It will be understood that the workpiece carrying tables on a typical two-axis contouring wire cutter electrical discharge machining machine, such as are shown in FIGS. 1, 2 and 3 of the drawings, would be capable of performing both straight line motion and continuous path contouring. Any of a number of commercial numerical tape control units are available for providing this type of operation. One example of such a control unit is the numerical tape control unit manufactured and sold by the Superior Electric Company of Bristol, Connecticut.
The invention of Applicants is particularly adapted for use with wire cutting electrical discharge machining and it takes into account the factors of thickness of the workpiece and material of the workpiece, which are important factors in determining the optimum on-off time ratio and the level to which the machining current will be reduced. This reduction is controlled through the pulse output of the off-time generator and to the short circuit control system which actually controls the machining power pulse off-time.